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<head><meta charset="utf-8"><title>gnzlbg minimal requirements · project-ffi-unwind · Zulip Chat Archive</title></head>
<h2>Stream: <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/index.html">project-ffi-unwind</a></h2>
<h3>Topic: <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html">gnzlbg minimal requirements</a></h3>

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<a name="178226074"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226074" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> nikomatsakis <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226074">(Oct 15 2019 at 19:24)</a>:</h4>
<blockquote>
<p><a href="https://github.com/gnzlbg/project-ffi-unwind" target="_blank" title="https://github.com/gnzlbg/project-ffi-unwind">https://github.com/gnzlbg/project-ffi-unwind</a></p>
</blockquote>
<p>this branch specifically? <a href="https://github.com/gnzlbg/project-ffi-unwind/tree/mvp_rfc" target="_blank" title="https://github.com/gnzlbg/project-ffi-unwind/tree/mvp_rfc">https://github.com/gnzlbg/project-ffi-unwind/tree/mvp_rfc</a></p>



<a name="178226187"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226187" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226187">(Oct 15 2019 at 19:25)</a>:</h4>
<p>Yes. That s just my current brain dump</p>



<a name="178226275"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226275" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226275">(Oct 15 2019 at 19:26)</a>:</h4>
<p>I was working with some thread cancellation issues in the UCGs and ended up needing this feature</p>



<a name="178226379"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226379" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226379">(Oct 15 2019 at 19:26)</a>:</h4>
<p>So I started down explaining my problem and what guarantees I need</p>



<a name="178226388"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226388" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> nikomatsakis <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226388">(Oct 15 2019 at 19:26)</a>:</h4>
<p>seems useful, yes</p>



<a name="178226392"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226392" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226392">(Oct 15 2019 at 19:26)</a>:</h4>
<p>Those are implementable on Linux at least</p>



<a name="178226411"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226411" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226411">(Oct 15 2019 at 19:26)</a>:</h4>
<p>There are some references to the relevant specs</p>



<a name="178226435"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226435" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> nikomatsakis <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226435">(Oct 15 2019 at 19:27)</a>:</h4>
<p>I spun this out into its own topic</p>



<a name="178226834"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226834" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226834">(Oct 15 2019 at 19:30)</a>:</h4>
<p>So for background: <a href="#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/pthread_cancel.20looks.20unsound" title="#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/pthread_cancel.20looks.20unsound">https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/pthread_cancel.20looks.20unsound</a> and <a href="https://github.com/rust-lang/rust-memory-model/issues/46" target="_blank" title="https://github.com/rust-lang/rust-memory-model/issues/46">https://github.com/rust-lang/rust-memory-model/issues/46</a></p>



<a name="178226899"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226899" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226899">(Oct 15 2019 at 19:30)</a>:</h4>
<p>Basically, on Linux (or POSIX) in general, pthreads has cancellation points on a lost of C library APIs, like open, sleep, and so on</p>



<a name="178226913"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178226913" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178226913">(Oct 15 2019 at 19:31)</a>:</h4>
<p>when a thread is cancelled, those functions unwind</p>



<a name="178227163"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227163" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227163">(Oct 15 2019 at 19:32)</a>:</h4>
<p>In the Itanium C++ ABI, there are ~3 types of unwinds, "native" (whatever the current programming language uses), "foreign" (what other programming languages that are not the current one uses), and "forced" (the stack frames do not control unwinding, but something external does, so the current stack frames cannot just stop unwinding completely, they can pause it, but must resume it - think <code>longjmp</code> where the stack frames being deallocated cannot stop it, only setjmp can, or thread cancellation..)</p>



<a name="178227286"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227286" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227286">(Oct 15 2019 at 19:34)</a>:</h4>
<p>I think this might be simpler in the C ABI (the x86-ps-abi), but a lot of the wording there about unwinding is identical to the itanium ABI spec (the itanium builds on that one, so they aren't really incompatible)</p>



<a name="178227342"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227342" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227342">(Oct 15 2019 at 19:35)</a>:</h4>
<p>So that's it, when you spawn a thread, and call <code>pthread_cancel</code> on its thread id, some <code>extern "C"</code> function in libc unwinds, and that's UB - I need to make those <code>"C unwind"</code> instead</p>



<a name="178227456"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227456" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227456">(Oct 15 2019 at 19:36)</a>:</h4>
<p>since we want to spawn new threads that start a Rust function, this is Rust (spawns thread)-&gt;C (pthread_create)-&gt;Rust (the callback that gets started)-&gt;C (a call to <code>open</code> or something that can cancel the thread)</p>



<a name="178227475"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227475" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227475">(Oct 15 2019 at 19:36)</a>:</h4>
<p>So I need Rust-&gt;C to work, where C unwinds with a non-Rust unwinding, over Rust frames</p>



<a name="178227505"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227505" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227505">(Oct 15 2019 at 19:36)</a>:</h4>
<p>Rust threads on Linux contain all kind of stuff, so the T: Copy restriction for stackframes isn't enough</p>



<a name="178227545"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227545" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227545">(Oct 15 2019 at 19:37)</a>:</h4>
<p>pthreads cancels threads with "forced unwinding", so <code>catch_unwind</code> needs to be implemented to ignore those on linux</p>



<a name="178227627"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227627" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227627">(Oct 15 2019 at 19:38)</a>:</h4>
<p>I have no idea what happens when <code>-C panic=abort</code> and "force unwinding"</p>



<a name="178227650"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178227650" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178227650">(Oct 15 2019 at 19:38)</a>:</h4>
<p>probably the frames are just deallocated like a <code>longjmp</code>, so that would be unsound over non <code>T: Copy</code> frames</p>



<a name="178323990"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178323990" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> acfoltzer <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178323990">(Oct 16 2019 at 20:08)</a>:</h4>
<p>yikes, I didn't realize that <code>pthread_cancel</code> does that, though it makes sense</p>



<a name="178324041"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178324041" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> acfoltzer <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178324041">(Oct 16 2019 at 20:09)</a>:</h4>
<blockquote>
<p>I have no idea what happens when <code>-C panic=abort</code> and "force unwinding"</p>
</blockquote>
<p>I know we don't emit landing pads with <code>panic=abort</code>, but is the <code>.eh_frame</code> still emitted I wonder? if not, I'm guessing the system unwinder would fail and abort the process</p>



<a name="178361995"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178361995" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178361995">(Oct 17 2019 at 08:52)</a>:</h4>
<p>So IIUC what happens with <code>-C panic=abort</code> is that we still unwind, but I don't know if destructors are run or not run consistently yet</p>



<a name="178362042"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362042" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362042">(Oct 17 2019 at 08:53)</a>:</h4>
<p>We probably could guard FFI calls to "C unwind" functions with code that causes an abort when <code>-C panic=abort</code></p>



<a name="178362114"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362114" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362114">(Oct 17 2019 at 08:54)</a>:</h4>
<p>But I think the real issue here is that when we say that it is UB for <code>extern "C"</code> to unwind, that requirement might be too strong</p>



<a name="178362130"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362130" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362130">(Oct 17 2019 at 08:54)</a>:</h4>
<p>since it would prevent <code>longjmp</code> on platforms where it is implemented by a particular kind of unwind, that LLVM supports even for <code>nounwind</code> functions</p>



<a name="178362136"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362136" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362136">(Oct 17 2019 at 08:54)</a>:</h4>
<p>and it would also prevent thread cancellation, which uses this exact same mechanism</p>



<a name="178362154"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362154" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362154">(Oct 17 2019 at 08:55)</a>:</h4>
<p>I think I would be fine with making this requirement for <code>extern "C"</code> functions</p>



<a name="178362164"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362164" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362164">(Oct 17 2019 at 08:55)</a>:</h4>
<p>that would require those functions to use <code>"C unwind"</code> instead</p>



<a name="178362214"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362214" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362214">(Oct 17 2019 at 08:56)</a>:</h4>
<p>as long as we allow "C unwind" functions to still unwind in some cases even when <code>-C panic=abort</code></p>



<a name="178362268"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362268" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362268">(Oct 17 2019 at 08:57)</a>:</h4>
<p>e.g. by allowing these to unwind with certain platform specific mechanisms as long as doing so does not break any safety invariants that rely on destructors being run, like <code>Pin</code>, or <code>crossbeam::scope</code></p>



<a name="178362287"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/210922-project-ffi-unwind/topic/gnzlbg%20minimal%20requirements/near/178362287" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> gnzlbg <a href="https://rust-lang.github.io/zulip_archive/stream/210922-project-ffi-unwind/topic/gnzlbg.20minimal.20requirements.html#178362287">(Oct 17 2019 at 08:57)</a>:</h4>
<p>for those kinds of unwind that would be allowed with <code>-C panic=abort</code>, whether destructors run or not run would be a platform-specific detail, that users need to look up</p>



<hr><p>Last updated: Aug 07 2021 at 22:04 UTC</p>
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